Mobile devices, such as cameras and mobile phones, are powered by batteries having limited energy and power. Eighty percent of current mobile phones have a built-in camera and flash and this number is expected to increase. The flash is used to provide light in dark and low-light ambient lighting situations. Modern camera algorithms are intelligently handling ambient lighting situations so that ambient light is used as background light in addition to the possibly needed extra light provided from flashes. The flashes are typically implemented by power LEDs (PLEDs) or xenon flashes. High current LEDs have lately been introduced for flashes in camera phones. LEDs are expected to approach the power levels of xenon in the near future and may replace built-in xenon flashes in many cameras. The major advantages of LEDs over xenon include low voltage operation, higher efficiency, and extreme miniaturization. In addition to the major advantages, an LED can produce continuous light (called “torch mode”) for a relatively long period, which is not possible for the xenon flash.
LEDs are current controlled devices, meaning that the intensity of their light output is proportional to the current passing through them. They also have a maximum current rating that may not be exceeded, otherwise the LEDs can be damaged. To limit the amount of current through an LED, a current-limiting resistor is typically inserted in series with the LED. If an LED is connected so that it is emitting light, a so-called forward voltage drop across the LED usually varies from 1.5V (volts) up to 5V, depending on the power output of the LED. The forward voltage drop is important in that the drop defines the voltage necessary in order to drive the LED. For example, a 1.5V battery will not be able to drive an LED with a 2.5V forward voltage drop. The LED will simply not emit light at the 1.5V level. In comparison, the main alternative technology for a flash in cameras and camera phones, the xenon flash, is a high voltage device, powered by high voltage instead of high current.
Drivers or charge storage devices are needed to drive LEDs or to source the peak power to flash LEDs. LEDs should only be connected directly to constant-voltage sources. Switched-mode power supplies (SMPSs) such as boost converters are used in some LED flashlights, stabilizing light output over a wide range of battery voltages and increasing the useful life of the batteries. SMPSs contain networks of energy storage inductors and capacitors and power handling electronic switches and rectifiers. A boost converter is a step-up DC (direct current) to DC converter with an output DC voltage greater than the input DC voltage. A charge pump can be also used to boost up the DC voltage but an extra charge balancing circuit may be needed. An LED has a positive and a negative terminal, also known as the anode and cathode. The cathode should be connected towards the ground or negative side of the driving voltage source, and the anode toward the positive side.
Although LEDs are proving to be useful for flashes, improvements in drivers for the LEDs could still occur.